罗圈里撞击坑铂族元素异常及粗铂矿的发现及其意义

对 1998年 7月发现的辽宁岫岩县苏子沟镇罗圈里直径 1.8km的撞击坑构造作了一系列地质、地球化学测试工作。从坑内出露的基岩 (下元古界辽河群里尔峪组变质砂岩 )与充填碎屑物质界面上取样 ,经XRD定性分析显示有自然铂、锇铱矿、自然铑、自然铱的存在。进一步对这一类样品作等离子质谱分析 (ICP MS) ,则发现有PGE异常。特别是Pt,Pd ,Ru甚至高于原始地幔的丰度值。进一步淘选坑里重砂样品则发现数颗粗铂矿物。已作电子探针分析表明PGE质量分数 :Pt为 85 .30 % 98.6 8% ,Pd为 0 .45 % 0 .87% ,Rh为 0 .5 6 % 0 .74% ,Os为 0 .6 6 % 1.31% ,Ir为 1.72 % 2 .43% ,Ru为 0 .16 % 0 .31% ,此外Ni为 0 .0 0 % 0 .5 0 % ,Fe为 4.98% 5 .42 %。笔者认为这种PGE元素异常和粗铂矿的发现是陨石及其撞击作用的重要依据之一。

THE DISCOVERY OF PGE ANOMALY AND PLATINA FROM LUOQUANLI IMPACT CRATER,CHINA

The authors have carried out a systematic geological investigation and geochemical analyses on the annular structure of the meteorite impact crater in Luoquanli of Xiuyan County, Liaoning Province, China, which is 1 8 km in diameter and was discovered in July of 1998. The samples were collected from the boundary between Quaternary cover and underlying Proterozoic basement within the crater. The XRD analyses showed the existence of Platinum, Iridosmine, Rhodium and Iridium. The ICP MS analyses indicated appreciable PGE anomaly the respective content of Pt, Pd and Ru even exceed their abundances in proto mantle. In addition, platina grains were revealed from panned samples. The composition of the platina given by EPMA is as follows (in mass percentage): Pt: 85 30% ~ 98 68%, Pd: 0 45% ~ 0 87%, Rh: 0 56% ~ 0 74%, Os: 0 66 %~ 1 31%, Ir: 1 72% ~ 2 43%, Ru: 0 16% ~ 0 31%, Ni: 0 00% ~0 50%, Fe: 4 98% ~ 5 42%. The authors consider the discovery of PGE anomaly and platina as significant evidence for confirming the meteorite impact event.